This pilot study will evaluate the safety and efficacy of interferon B in the treatment of polyarticular juvenile rheumatoid arthritis (JRA), and to establish a dosing range for this medication in the treatment of polyarticular JRA. JRA is an incurable idiopathic condition that affects at least 70,000 children in the United States (Cassidy JT, Nelson AM, The frequency of juvenile arthritis. J Rheumatol 1988; 15:535). Polyarticular disease, which makes up at least 1/3 of this number, is one of the most severe subtypes owing to its tendency to cause pain and damage of multiple joints. A recent summary of outcomes in polyarticular JRA reported that 45% of children with this form of arthritis have active disease 10 years after onset of symptoms, and 54% have radiographic evidence of joint damage (Levinson JE, Wallace CA, J Rheumatol 1992; 19:6). Although new medications-especially methotrexate-appear to have improved the prognosis of children with polyarticular JRA, a uniformly safe and effective therapy is elusive. Interferon B is an ideal agent to test as a novel therapy for polyarticular JRA. Large scale use in patients of all ages with autoimmune disease has documented its safety, and it inhibits many of the processes thought to be central to the pathogenesis of joint inflammation in JRA (Cirell R, Tyring SK. Major therapeutic use of interferons. Clin Immunother 1995; 3:27-87). Subjects will receive each of two dosages by intramuscular injection once weekly of interferon B for 12 weeks each in a cross-over fashion. The order in which each patient receives each dose will be randomized, and both patient and physician will be blinded to the dose being administered.